Support drivers other than PostgreSQL

[albireox]

Right now it's hardcoded in sdssdb that all the databases are PostgreSQL. In principle we may need to support other databases (e.g. SQLite or SQL Server) and should be able to use all the current framework. Both Peewee and SQLA support multiple drivers.

On the SQLA side this seems relatively simple, since the driver is specified as a string. For Peewee is a bit more complicated; we'd need to create a PeeweeBaseDatabaseConnection and then subclass from the appropriate Peewee connection class and make sure the __init__ is run correctly.